This research is aimed at extending our knowledge of the structural, functional and metabolic relationships of the various isoferritins we have recently found in many mammalian organs. Present evidence indicates that many of these multiple forms represent hybrid molecules fashioned from different amounts of dissimilar subunits. Detailed structural analyses will be made of isoferritins in human liver and horse spleen ferritin to define more clearly the molecular basis for the multiple forms. These ferritins will be separated into their constituent isoferritins by ion exchange chromatography or preparative isoelectric focusing. Individual isoferritins will be analyzed according to their amino acid composition, subunit composition and cyanogen bromide peptide profiles. Reconstitution experiments will be performed to investigate possible requirements and relationships of the different subunits in the structure and function of apoferritin. Comparative studies will be made of tryptic and cyanogen bromide peptide profiles of isoferritins from these and other mammalian species to investigate possible invariant regions in the molecule that may be related to function. In related experiments, investigations will be made into metabolic relationships of the iron and protein components of the isoferritins in rat liver, spleen and heart. The ability of the different isoferritins in a tissue to accept and release iron will be studied in vivo and in vitro. The relative rates of synthesis and turnover of the protein shells of the various isoferritins will be studied in vivo under different modalities of iron therapy and with different iron compounds by tracer techniques with radioactive amino acids. Attempts will also be made to investigate regulatory mechanisms operating in the synthesis of tissue isoferritins by use of homologous and heterologous cell-free systems capable of synthesizing complete apoferritin subunits.